[en] The present consideration of performance capabilities and system design requirements for a Brayton cycle conversion system usable by future Ariane 5 launch vehicle applications gives attention to such a power system's matching to available radiator concepts and dimensions, the use of direct or indirect waste heat transfer to the radiator and of simple or intercooled cycles, as well the consequences of gas cycle selection on reactor technology. The results presented are expected to be useful in the optimization of a 20-30 kW(e) system employing a liquid metal-cooled nuclear reactor in conjunction with a gas turbine energy conversion system. 10 references

[en] The specifications of permeation barriers, tritium recovery process maintaining a very low tritium activity in the coolant, and control of the coolant chemistry, required the evaluation of the tritium losses through the steam generators and include the definition of its operating conditions by thermodynamic cycle calculations and its thermal-hydraulic design. For both tasks specific computer tools were developed. The obtained geometry, surface area, and temperature profiles along the heat exchanger tubes were then used to estimate the daily tritium permeation into the steam cycle. Steam oxidized Incoloy 800 austenitic stainless steel was identified as the best suited existing material; in nominal steady-state operation, the tritium escape into the steam cycle could be restricted to less than 10 Ci/d. Tritium permeation during temperature and pressure transients in the steam generator (destruction and possible self-healing of the permeation barrier) is identified to bear a large tritium release potential. Solutions are proposed. (from authors). 4 figs., 1 tab

[en] The tritium cycle of a fusion reactor is hereafter investigated by a synthetic model of the tritium circulation between the blanket, the tritium recovery units from the breeder, the coolant, the plasma exhaust and the storage unit. Analytical expressions of the minimum required breeding capability and of the initial tritium supply are derived to analyse the sensitivity of these crucial parameters to the fractional burn up, to the tritium losses (radioactive and others) and to the processing time associated with the various units. As confirmed by the parametric study of a few typical situations, the necessary breeding capability and the initial tritium supply are essentially functions of the total equilibrium inventory. In addition, the distribution of this total inventory among the various units and the possible disproportion of the time scales required by different recovery processes, strongly influence the initial tritium requirement and the doubling time associated with given breeding performances

[en] Recent CNES/CEA prospective studies of an orbit transfer vehicule to be launched by ARIANE V, emphasize the advantage of the Brayton cycle over the thermionics and thermoelectricity, in minimizing the total mass of 100 to 300 kWsub(e) power systems under the constraint specific to ARIANE of a radiator area limited to 95 m². The review of candidate reactor concepts for this application, finally recommends both liquid metal and gas cooled reactors, for their satisfactory adaptation to a reference Brayton cycle and for the available experience from the terrestrial operation of comparable systems

[en] A cooperative program about space nuclear turboelectric space power systems was initiated in 1982 by both the Centre National d'Etudes Spatiales (CNES) and the Commissariat a l'Energie Atomique (CEA) agencies of the French government, with a view to assessing the feasibility, the lead time and the cost for the development of nuclear space power systems relevant to the power range of 20 to 400 kWe. The effort on conceptual studies recently shifted towards low power systems (20 kWe). In order to widely cover the range of possible technologies for the 20-kWe space power systems, and to assess the impact of the reactor concept (liquid metal versus gas cooled) and of the relevant operating temperature upon the system performances, a set of three reference turboelectric systems were selected for comparison. Those considered for both extreme bounds of the explored range of maximum heat source temperature (970 K and 1470 K), use a liquid metal cooled reactor (sodium or Nak and lithium respectively) and the basic features of the 200 kWe system. In addition to both liquid metal cooled systems, a third system using a gas cooled epithermal particle bed reactor, to drive a direct cycle conversion system with a turbine inlet temperature of 1120 K is being investigated; this system is intended to make full use of the heat resisting materials and of the techniques, which have been developed for the high temperature gas cooled reactors

[en] Within the framework of a European Community programme aiming at the development of a water-cooled lithium-lead (Pb17Li) blanket concept for a demonstration fusion reactor, important experimental works are devoted to the development of efficient tritium permeation barriers. Among them, in-pile tests (LIBRETTO experiments) with closed capsules containing stagnant Pb17Li are intended to assess under irradiation the efficiency of these barriers. In this paper, a method for interpreting LIBRETTO data is presented, which relies on the use of a 2-dimensional finite element simulation model exploiting the analogy between tritium migration and heat conduction. Preliminary results obtained with this model are also discussed, which seem to indicate that, under conditions comparable with the experiment, the permeation barrier tested in LIBRETTO-2 would reduce tritium permeation by a factor of about 80

[en] Given the variety of possible missions and flight dates, it seems advisable to widen the basis for future technical choices within the French preliminary studies of 20-kWe space nuclear power systems. In addition to the fast spectrum, liquid metal-cooled reactor presently considered as a reference, shorter development term system, gas- and Na(K)-cooled thermal spectrum reactors are being investigated. The need for adequate ZrH moderator temperature conditions can be satisfied through a Brayton cycle conversion subsystem featuring two separate, high temperature-heat pipes and low temperature-pumped loop radiators. The penalty in efficiency and in radiator area, resulting from the wanted lower reactor inlet temperature, can be limited, particularly in the case of the higher temperature, gas-cooled reactor system. A multiple, pivoting tubes, low temperature radiator concept is proposed; it avoids an extension of the related structural support frame beyond the conversion subsystem region in flight configuration. Arrangements peculiar to small reactors and two-turbo-generator diagrams for reliability reasons are presented. Provisional, not yet optimized, thermal management mass estimates are evaluated

[en] A companion paper (Carre et al. 1989) presented at this symposium gives an overview of the French preliminary studies on space nuclear power systems in progress within the framework of a three-year (1986-1989) program. Other papers (Proust et al. 1988, Tilliette et al. 1988, Tilliette IECEC 1988) supplement the information on this activity. Low power levels of about 20-KWe and both liquid metal- and gas-cooled reactors are concerned. The Brayton cycle is currently selected as the conversion subsystem. Critical issues like safety, reliability, radiation shielding and reactor concept and technology have to be addressed more and more carefully and relevant temperature conditions are crucial. It is shown in this paper that the Brayton cycle can offer a valuable flexibility which allows the desired thermal environment. For instance, it is possible to significantly decrease the reactor inlet temperature and consequently, also given an adequate design, to favourably put forward convenient solutions for the lateral and axial bottom reflector, the shadow shield, the control drums drives and safety rods actuators and penetrations as well as for the possibility of using efficient moderator materials like metal hydrides (ZrH or ⁷LiH), which is worth being investigated as far as low power levels are concerned. Examples of Brayton cycle conversion subsystems and possible reactor arrangements are presented for both gas-cooled and liquid metal (NaK or Na)-cooled reactor heat sources. The study follows up the research described by Thilliette (1988, IECEC)

[en] Within the framework of a joint program initiated in 1983 by the two French Government Agencies C.N.E.S. (Centre National d'Etudes Spatiales) and C.E.A. (Commissariat a l'Energie Atomique), in order to study space nuclear power systems for future ARIANE 5 applications, extensive investigations have dealt with the Brayton cycle which has been selected as the energy conversion system. Several aspects can be mentioned in this field: the matching of the power system to the available radiator dimensions up to 200 kWe, the direct or indirect waste heat transfer to the radiator, the use of a recuperator, the recent work on moderate (25 kWe) power levels, the simulation studies related to various operating conditions and the general system optimization. A limited experimental program is starting on some crucial technology areas including a first contract to the industry concerning the turbogenerator. Particular attention is being paid to the significance of the adoption of a Brayton cycle for space applications involving a nuclear heat source which can be either a liquid metal-cooled or a gas-cooled reactor. As far as a gas-cooled reactor, direct cycle system is concerned, the relevance to the reactor technology and the concept for moderator thermal conditioning, is particularly addressed

[en] The tritium cycle of a fusion reactor is hereafter investigated by a synthetic model of the tritium circulation between the blanket, the tritium recovery units from the breeder, the coolant, the plasma exhaust and the storage unit. Analytical expressions of the minimum required breeding capability and of the initial tritium supply are derived to analyse the sensitivity of these crucial parameters to the fractional burn up, to the tritium losses (radioactive and others) and to the processing time associated with the various units. As confirmed by the parametric study of a few typical situations, the necessary breeding capability and the initial tritium supply are essentially functions of the total equilibrium inventory. In addition, the distribution of this total inventory among the various units and the possible disproportion of the time scales required by different recovery processes, strongly influence the initial tritium requirement and the doubling time associated with given breeding performances